Process for preparing protective-colloid-stabilized vinylaromatic-1,3-diene copolymers

ABSTRACT

A method is provided for producing protective colloid-stabilized vinyl aromatic-1.3-diene-copolymers in the form of their aqueous polymer dispersions or in the form of a powder which can be re-dispersed in water, by emulsion-polymerizing a mixture containing at least one vinyl aromatic and at least one 1,3-diene in the presence of a protective colloid and optionally, drying the resulting polymer dispersions. In the method part of the protective colloid is provided straightaway and part is metered.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The invention relates to a process for preparingprotective-colloid-stabilized vinylaromatic-1,3-diene copolymers.

2) Background Art

Protective-colloid-stabilized polymers, especially in the form of theiraqueous dispersions or in the form of their water-redispersible polymerpowders, are used in a wide variety of applications, for example ascoating compositions or adhesives for a very wide variety of substrates.Polymers whose preparation in aqueous media, for example by emulsionpolymerization, involves stabilization by emulsifiers, tend to have highwater absorption when used, due, to the emulsifier content. When theyare used as an adhesive, for example, they have reduced bonding power.

These disadvantages can be overcome by stabilizing the polymerizationmixture exclusively with protective colloids. Theprotective-colloid-stabilized aqueous polymer dispersions andprotective-colloid-stabilized polymer powders which have so far becomeestablished are especially those based on vinyl ester homopolymers orvinyl ester copolymers, specifically polyvinyl acetate or vinylacetate-ethylene copolymers. The reason for this is that the stabilizingeffect developed by protective colloids is generally lower than that ofemulsifiers, so that complete replacement of emulsifiers by protectivecolloids has hitherto given satisfactory products only with relativelyhydrophilic polymers, such as the abovementioned vinyl ester polymers.

However, polymers with any hydrophobic character, such asstyrene-butadiene copolymers, are preferred in many applications, andthe polymerization of polymers of this type requires effectivestabilization. Styrene-butadiene copolymers are therefore generallypolymerized in an aqueous phase in the presence of emulsifiers. Thehydrophilic character of the emulsifier content present, however,counteracts to some extent the hydrophobic properties of the copolymerwhen emulsifier-stabilized styrene-butadiene copolymers are used.

In particular when polymers are used in the form of their redispersionpowders, for improving mortar properties, a main application sector forredispersion powders, the formulations have to remain stable for acertain time and must not change their working consistency significantly(cement stability). This is because the user cannot be expected to remixat frequent intervals. In the concrete and mortar industry a significantrole is played by mechanical properties, such as compressive strengthand porosity, and the associated air pore content. If too many air poresare present there is a severe reduction in compressive strength, and iftoo few or no air pores are present in the mortar or concrete, thebuilding material has insufficient resistance to frost and condensation.In addition, the hydraulically setting systems modified with thedispersion powder should provide adhesion which is even better than thatof unmodified systems.

WO-A 96/17891 discloses water-redispersible dispersion powders based onstyrene-butadiene copolymers. The powders are obtainable by emulsionpolymerization in the presence of emulsifiers. Prior to drying,monosaccharide, polyvinylpyrrolidone and, if desired, emulsifier arethen added to the dispersion. WO-A 96/20963 discloses a process forpreparing water-redispersible polymer powders based on styrene-butadienepolymers. The polymers here are prepared by two-stage polymerization inthe presence of emulsifier to give core-shell polymers, and are spraydried. WO-A 96/41825 likewise relates to dispersion powders based oncore-shell polymers. The shell here has saccharide-functional comonomersand crosslinkable comonomers for covalent linking of the shell to thecore. In addition to the relatively complicated procedure for preparingthe redispersion powders, these powders have the disadvantages discussedabove of emulsifier-stabilized styrene-butadiene copolymers, and inaddition they have unsatisfactory performance, specifically ease of usewith regard to cement stability.

WO-A 97/15603 relates to protective-colloid-stabilized emulsion polymersof conjugated dienes, with mercaptosilanes copolymerized to improve theprotective-colloid action. Disadvantages here are that thepolymerization uses emulsifiers as well as the protective-colloid andespecially that the copolymerization of mercaptosilane, which is anessential requirement, makes the preparation more expensive and isundesirable for many applications since it reduces the degree ofcrosslinking of the butadiene units.

EP-A 538571 relates to a process for preparingprotective-colloid-stabilized polymer dispersions based on polymershaving more than 50% content of styrene and/or of (meth)acrylate. All ofthe polyvinyl alcohol used for stabilization is used as an initialcharge, and specific initiator systems are used to adjust hydrophilicproperties and viscosity. In the examples, stable dispersions areobtained with styrene, butyl acrylate and acrylamide. A disadvantage isthat this process does not give stable dispersions from thecopolymerization of styrene and butadiene.

EP-B 62106 discloses a procedure for preparingpolyvinyl-alcohol-stabilized polymer dispersions based on styrene unitsand/or on (meth)acrylate units. The minimum temperature for theprocedure is above 65° C., and it is carried out in the presence oforganic initiators or peroxosulphur compounds, an by feeding the mainportion of the monomers. The preparation of styrene-butadiene copolymersis not dealt with.

U.S. Pat. No. 5,200,459 describes a process for preparingpolyvinyl-alcohol-stabilized butadiene copolymers, in the presence of awater-miscible solvent to ensure dispersion stability.

DE-A 4212768 relates to a process for preparing synthetic-polymerdispersions, inter alia styrene-butadiene copolymer dispersions. Theseare prepared without addition of emulsifier, but instead of this in thepresence of a copolymerizable macromonomer based on crosslinkingproducts of polyalkylene glycols and maleic acid or fumaric acid.

U.S. Pat. No. 4,299,903 teaches the preparation of an emulsifier-freetoner resin based on styrene-butadiene by copolymerization in thepresence of a “charge control agent” selected from the group consistingof quaternary ammonium salts or alkylpyridinium compounds.

The object of the invention was therefore to provide a process which cangive access to protective-colloid-stabilized vinylaromatic-1,3-dienecopolymers without addition of other auxiliaries to stabilize thepolymerization mixture. The products of the process should have entirelysatisfactory cement stability, in particular when used with hydraulicbinders.

SUMMARY OF THE INVENTION

The invention provides a process for preparingprotective-colloid-stabilized vinylaromatic-1,3-diene copolymers in theform of their aqueous polymer dispersions or in the form of theirwater-redispersible powders, by emulsion polymerization of a mixturecomprising at least one vinylaromatic and comprising at least one1,3-diene in the presence of protective colloid, and, if desired, dryingthe resultant polymer dispersions, characterized in that some of theprotective colloid is used as an initial charge and some of theprotective colloid is used as a feed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Suitable vinylaromatics are styrene and methylstyrene, preferablystyrene. Examples of 1,3-dienes are 1,3-butadiene and isoprene,preferably 1,3-butadiene. The polymerization is generally carried outusing mixtures with from 20 to 80% by weight of vinylaromatic and from20 to 80% by weight of 1,3-diene, and the mixtures may, if desired, alsocomprise further monomers and the per cent by weight data always give atotal of 100% by weight.

Based on the total weight of the monomer phase, up to 30% by weight ofother monomers copolymerizable with vinylaromatics and with 1,3-dienesmay also be polymerized, for example ethylene, vinyl chloride,(meth)acrylates of alcohols having from 1 to 15 carbon atoms or vinylesters of unbranched or branched carboxylic acids.

Based on the total weight of the monomer mixture, from 0.05 to 10% byweight of auxiliary monomers may also be copolymerized. Examples ofauxiliary monomers are ethylenically unsaturated mono- and dicarboxylicacids, preferably acrylic acid, methacrylic acid, fumaric acid andmaleic acid; ethylenically unsaturated carboxamides and carbon itriles,preferably acrylamide and acrylonitrile; mono- and diesters of fumaricor maleic acid, such as the diethyl and diisopropyl esters, and alsomaleic anhydride, ethylenically unsaturated sulphonic acids and salts ofthese, preferably vinylsulphonic acid, and2-acrylamido-2-methylpropanesulphonic acid. Other examples areprecrosslinking comonomers, such as unsaturated comonomers having two ormore ethylenic unsaturated double bonds, e.g. divinyl adipate, diallylmaleate, allyl methacrylate or triallyl cyanurate, or postcrosslinkingcomonomers, e.g. acrylamidoglycolic acid (AGA), methylmethacrylamidoglycolate (MAGME), N-methylolacrylamide (NMA),N-methylolmethacrylamide, allyl N-methylolcarbamate, alkyl ethers, suchas the isobutoxy ether, or esters of N-methylolacrylamide, ofN-methylolmethacrylamide, or of allyl N-methylol carbamate. Othersuitable auxiliary monomers are epoxy-functional comonomers, such asglycidyl methacrylate or glycidyl acrylate. Other examples of auxiliarymonomers are silicon-functional comonomers other than mercapto silanes,e.g. acryloxypropyltri(alkoxy)- and methacryloxypropyltri(alkoxy)silanes, vinyltrialkoxysilanes andvinylmethyldialkoxysilanes, and examples of alkoxy groups which may bepresent here are ethoxy ether radicals and ethoxypropylene glycol etherradicals. Mention may also be made of monomers having hydroxyl or COgroups, such as hydroxyalkyl (meth)acrylates, e.g. hydroxyethyl,hydroxypropyl or hydroxybutyl (meth)-acrylate, and also compounds suchas diacetone-acrylamide and acetylacetoxyethyl(meth)acrylate.

The selection of monomers and the selection of the proportions by weightof the comonomers here takes place in such a way that the resultantglass transition temperature Tg is generally from −50° C. to +50° C.,preferably from −20° C. to +40° C. The glass transition temperature Tgof the polymers may be determined in a known manner by differentialscanning calorimetry (DSC). The Tg may also be approximated by the Foxequation. According to T. G. Fox, Bull. Am. Physics Soc. 1, 3, page 123(1956): 1/Tg=x₁/Tg₁+x₂/Tg₂+ . . . +x_(n)/Tg_(n), where x_(n) is theproportion by weight (% by weight/100) of monomer n and Tg_(n) is theglass transition temperature in degrees Kelvin of the homopolymer ofmonomer n. Tg values for homopolymers are listed in Polymer Handbook2^(nd) Edition, J. Wiley & Sons, New York (1975).

The protective-colloid-stabilized polymers are prepared by emulsionpolymerization, generally with a polymerization temperature of from 40to 100° C., preferably from 60 to 90° C. When gaseous comonomers, suchas ethylene or vinyl chloride, are copolymerized it is also possible tooperate under pressure, generally between 5 bar and 100 bar.

The polymerization is initiated with the redox-initiator combinations orinitiators usually used for emulsion polymerization. Examples ofsuitable organic initiators are hydroperoxides, such as tert-butylhydroperoxide, tert-butyl peroxopivalate, cumene hydroperoxide, orisopropylbenzene monohydroperoxide, and azo compounds, such asazobisisobutyronitrile. Suitable inorganic initiators are the sodium,potassium and ammonium salts of peroxodisulphuric acid. The initiatorsmentioned are generally used in amounts of from 0.05 to 3% by weight,based on the total weight of the monomers.

The redox initiators used are combinations of the initiators mentionedwith reducing agents. Suitable reducing agents are the sulphites andbisulphites of alkaline metals and of ammonium, for example sodiumsulphite, the derivatives of sulphoxylic acid, such as theformaldehyde-sulphoxylate of zinc or of an alkaline metal, e.g. sodiumhydroxymethanesulphinate, and ascorbic acid. The amount of reducingagent is preferably from 0.01 to 5.0% by weight, based on the totalweight of the monomers.

To control the molecular weight, regulating substances may be usedduring the polymerization. They are usually used in amounts of from 0.01to 5.0% by weight, based on the monomers to be polymerized, and are fedseparately or else dosed in a form premixed with reaction components.Examples of substances of this type are n-dodecyl mercaptan,tert-dodecyl mercaptan, mercaptopropionic acid, methyl mercaptopropionate, isopropanol and acetaldehyde.

The polymerization mixture is stabilized by protective colloids, with noemulsifiers added. Suitable protective colloids are fully or partiallyhydrolysed polyvinyl acetates. Partially hydrolysed hydrophobicizedpolyvinyl acetates are also suitable, and the hydrophobicization may,for example, take place by copolymerization with isopropenyl acetate,ethylene or vinyl esters of saturated alpha-branched monocarboxylicacids having from 5 to 11 carbon atoms. Other examples of protectivecolloids are polyvinyl pyrrolidones; polysaccharides in a water-solubleform, such as starches (amylose and amylopectin), celluloses and theircarboxymethyl, methyl, hydroxyethyl or hydroxypropyl derivatives;proteins, such as casein or caseinate, soy protein, gelatine;ligninsulphonate; synthetic polymers, such as poly(meth)acrylic acid,copolymers of (meth)acrylates with carboxy-functional comonomer units,poly(meth)acrylamide, polyvinyl sulphonic acids and water-solublecopolymers of these; melamine-formaldehydesulphonates,napthaleneformaldehydesulphonates, styrene-maleic acid copolymers, vinylether-maleic acid copolymers, and dextrins, such as yellow dextrin.

Preference is given to said partially hydrolysed polyvinyl acetates andpartially hydrolysed hydrophobicized polyvinyl acetates. Particularpreference is given to partially hydrolysed polyvinyl acetates with adegree of hydrolysis of from 80 to 95 mol % and with a Höppler viscosity(4% strength aqueous solution, DIN 53015, Höppler method at 20° C.) offrom 1 to 30 mPas, preferably from 2 to 15 mPas.

The amount of the protective colloids added during the polymerization isgenerally from 1 to 15% by weight in total, based on the total weight ofthe monomers. Some of this content of protective colloid is an initialcharge and some is fed after initiating the polymerization. From 2 to90% by weight, preferably from 30 to 85% by weight, of the protectivecolloid content is usually used as initial charge, based in each case onthe total weight of the protective colloid content, and the remainder isfed. A significant factor in deciding the proportion by weight ofcollective colloid to be used as an initial charge here is the contentof monomer in the initial charge. The initial charge and the feed of theprotective colloid here are adjusted so that the protective colloidratio, based on the content of monomer in the initial charge, is from0.1:1 to 0.9:1. During the polymerization monomer and protective colloidare then fed in such a way that the amount of protective colloid presentafter the polymerization has ended is from 1 to 15% by weight in total,based on the total weight of the monomers.

The monomers may be used entirely as an initial charge, entirely as afeed, or some proportion may be used as an initial charge and theremainder fed after the polymerization has been initiated. A preferredprocedure is to use from 10 to 25% by weight, based on the total weightof the monomers, as an initial charge and to feed the remainder. Thefeeds may be separate (spatially and chronologically). Some or all ofthe components to be fed may be fed in preemulsified form. To initiatethe polymerization the thermal initiator may be entirely within theinitial charge, or partly within the initial charge and partly fed, orexclusively fed.

Once the polymerization is complete, postpolymerization may be carriedout by known methods to remove residual monomer, for example usingredox-catalyst-initiated postpolymerization. Volatile residual monomersmay also be removed by distillation, preferably at reduced pressure,and, if desired, by passing inert carrier gases, such as air, nitrogenor water vapour, through or over the product.

The aqueous dispersions obtainable by the process according to theinvention have a solids content of from 30 to 75% by weight, preferablyfrom 40 to 65% by weight. To prepare the water-redispersible polymerpowders the aqueous dispersions are dried, for example by fluidized-beddrying, freeze drying or spray drying. The dispersions are preferablyspray dried. The spray drying takes place in conventional spray dryingsystems, with atomization by single-, twin- or multifluid nozzles or bya rotating disc. The discharge temperature is generally chosen withinthe range from 55 to 100° C., preferably from 70 to 90° C., depending onthe system, the Tg of the resin and the desired degree of drying.

The total amount of protective colloid prior to the drying procedureshould preferably be at least 10% by weight, based on the polymercontent. To ensure redispersibility it is generally necessary to addfurther protective colloids to the dispersion prior to the drying, asspraying aids. The amount of spraying aids usually used is from 5 to 25%by weight, based on the polymeric constituents of the dispersion.

Suitable spraying aids are partially hydrolysed polyvinyl acetates;polyvinylpyrrolidones; poly saccharides in a water-soluble form, such asstarches, (amylose and amylopectin), celluloses and their carboxymethyl,methyl, hydroxyethyl or hydroxypropyl derivatives; proteins, such ascasein or caseinate, soy protein, gelatines, ligninsulphonates,synthetic polymers, such as poly(meth)acrylic acid, copolymers of(meth)acrylates with carboxy-functional comonomer units,poly(meth)acrylamide, polyvinylsulphonic acids and water-solublecopolymers of these; melamine-formaldehydesulphonates,naphthaleneh-formaldehyde sulphonates, styrene-maleic acid copolymersand vinyl ether-maleic acid copolymers. The spraying aids whose use ispreferred are partially hydrolysed polyvinyl acetates with a degree ofhydrolysis of from 80 to 95 mol %, with a Höppler viscosity of from 1 to30 mPas, and these may, if desired, have been modified with isopropenylacetate units or with vinyl ester units.

The content of up to 1.5% by weight of antifoam, based on the basepolymer, has frequently proved advantageous during spraying. To increasestorage stability by improving blocking resistance, in particular in thecase of powders with a low glass transition temperature, the powderobtained may be mixed with an antiblocking agent (anticaking agent),preferably up to 30% by weight, based on the total weight of polymericconstituents. Examples of antiblocking agents are calcium carbonate,magnesium carbonate, talc, gypsum, silica and silicate with particlesizes preferably within the range from 10 nm to 10 μm.

To improve performance, other additives may be added during spraying.Examples of other constituents present in preferred embodiments ofdispersion powder compositions are pigments, fillers, foam stabilizers,and hydrophobicizing agents.

The water-redispersible, protective-colloid-stabilized polymer powdersmay be used in the application sectors in which these are typicallyused.

The examples below serve to explain the invention further:

EXAMPLE 1

1110 ml of deionized water and 655 g of a 20% strength aqueous solutionof a partially hydrolysed polyvinyl acetate with a degree of hydrolysisof 88 mol % and with a Höppler viscosity of 4 mPas for the 4% strengthsolution (DIN 53015, Höppler method at 20° C.) formed an initial chargein a stirred autoclave of about 5 l capacity. The pH was adjusted to4.0-4.2 using 10% strength by weight formic acid. This was followed byevacuation, nitrogen flushing, reevacuation and introducing into theevacuated vessel a mixture made from 112 g of styrene, 168 g of1,3-butadiene and 8 g of tert-dodecyl mercaptan. After heating to 80° C.the polymerization was initiated by simultaneously running in twocatalyst solutions. The first of these was composed of 110 g ofdeionized water and 15.5 g of a 40% strength aqueous tert-butylhydroperoxide solution. The other was composed of 116 g of deionizedwater and 13 g of sodium formaldehydesulphoxylate. The two catalystsolutions were fed at the same rate (18 ml/h). Once the polymerizationhad started the feed of a mixture of 951 g of 1,3-butadiene, 634 g ofstyrene and 9 g of tert-dodecyl mercaptan was begun at 5.3 g/min. Therewas a simultaneous feed of 245 g of a 20% strength by weight aqueoussolution of a partially hydrolysed polyvinyl acetate with a degree ofhydrolysis of 88 mol % and with a Höppler viscosity of 4 mPas for the 4%strength solution (DIN 53015, Höppler method at 20° C.) at 0.82 g/min.Once the monomer feed and polyvinyl alcohol feed had ended,postpolymerization was carried out for 2 h at 80° C. with an unchangedinitiator solution feed rate, and then the initiator solution feed wasterminated. The mixture was then cooled. This gave a stable,coarse-particle (Coulter LS 230; Dw =2.54 m) coagulate-free dispersionwith a solids content of 49.4% and a viscosity (Brookfield viscometer,20° C., 20 rpm) of 270 mPas.

400 parts by weight of the dispersion were mixed thoroughly with 200parts by weight of a 10.3% strength by weight solution of a polyvinylalcohol (partially hydrolysed polyvinyl acetate, degree of hydrolysis 88mol %, viscosity of the 4% strength solution 13 mPas), 0.84 parts byweight of antifoam and 135 parts by weight of water. The dispersion wassprayed through a twin-fluid nozzle. The spraying component used wascompressed air at 4 bar, and the droplets formed were dried withcocurrent air heated to 125° C. The resultant dry powder was mixed with10% of commercially available antiblocking agent (a mixture of calciummagnesium carbonate and magnesium hydro-silicate).

EXAMPLE 2

The dispersion was prepared as in Example 1 using a total of 900 g of a20% strength by weight aqueous solution of a partially hydrolysedpolyvinyl acetate with a degree of hydrolysis of 88 mol % and with aHöppler viscosity of 4 mPas for the 4% strength solution. 468 g of theaqueous polyvinyl alcohol solution here were used as an initial chargein the pressure reactor and the remaining 432 g were used as a feedduring the polymerization as described in Example 1. This gave a stable,coarse-particle (Coulter LS 230; Dw=2.73 μm) coagulate-free dispersionwith a solids content of 49.9% and a viscosity (Brookfield viscometer20° C., 20 rpm) of 260 mPas. The action taken to prepare the dispersionpowder was as in Example 1.

EXAMPLE 3

The dispersion was prepared as in Example 1 using a total of 900 g of a20% strength by weight aqueous solution of a partially hydrolysedpolyvinyl acetate with a degree of hydrolysis of 88 mol % and with aHöppler viscosity of 4 mPas for the 4% strength solution. 281 g of theaqueous polyvinyl alcohol solution here were used as an initial chargein the pressure reactor and the remaining 619 g were used as a feedduring the polymerization as described in Example 1. This gave a stable,coarse-particle (Coulter LS 230; Dw=2.63 μm) coagulate-free dispersionwith a solids content of 49.8% and a viscosity (Brookfield viscometer20° C., 20 rpm) of 18,400 mPas. All of the other actions taken were asin Example 1.

EXAMPLE 4

The dispersion was prepared as in Example 1 using a total of 900 g of a20% strength by weight aqueous solution of a partially hydrolysedpolyvinyl acetate with a degree of hydrolysis of 88 mol % and with aHöppler viscosity of 4 mPas for the 4% strength solution. 720 g of theaqueous polyvinyl alcohol solution here were used as an initial chargein the pressure reactor and the remaining 180 g were used as a feedduring the polymerization as described in Example 1. This gave a stable,coarse-particle (Coulter LS 230; Dw=2.60 μm) coagulate-free dispersionwith a solids content of 49.0% and a viscosity (Brookfield viscometer20° C., 20 rpm) of 300 mPas. All of the other actions taken were as inExample 1.

EXAMPLE 5

The dispersion was prepared as in Example 1 using a total of 1000 g of a20% strength by weight aqueous solution of a yellow dextrin (Avedex 35).500 g here were used as an initial charge in the pressure reactor andthe remaining 500 g were used as a feed during the polymerization asdescribed in Example 1. This gave a stable, coarse-particle (Coulter LS230; Dw=2.80 μm) coagulate-free dispersion with a solids content of49.8% and a viscosity (Brookfield viscometer 20° C., 20 rpm) of 310mPas. All of the other actions taken were as in Example 1.

EXAMPLE 6

The dispersion was prepared as in Example 1, using 37 g of acrylic acidas an initial charge and using a total of 1000 g of a 20% strength byweight aqueous solution of hydroxypropylcellulose (Kucel L), of which500 g were used as an initial charge in the pressure reactor and theremaining 500 g were used as a feed during the polymerization asdescribed in Example 1. This gave a stable, coarse-particle (Coulter LS230; Dw=2.74 μm) coagulate-free dispersion with a solids content of48.0% and a viscosity (Brookfield viscometer 20° C., 20 rpm) of 420mPas. All of the other actions taken were as in Example 1.

EXAMPLE 7

The dispersion was prepared as in Example 1, using 37 g of acrylic acidas an initial charge and using a total of 1000 g of a 20% strength byweight aqueous starch solution (Nylgum), of which 500 g were used as aninitial charge in the pressure reactor and the remaining 500 g were usedas a feed during the polymerization as described in Example 1. This gavea stable, coarse-particle (Coulter LS 230; Dw=2.86 μm) coagulate-freedispersion with a solids content of 48.4% and a viscosity (Brookfieldviscometer 20° C., 20 rpm) of 500 mPas. All of the other actions takenwere as in Example 1.

Comparative Example 1

The dispersion was prepared as in Example 1, the entire amount of 900 gof the 20% strength by weight aqueous solution of the partiallyhydrolysed polyvinyl acetate with a degree of hydrolysis of 88 mol % andwith a Höppler viscosity of 4 mPas for the 4% strength solution beingused as an initial charge in the pressure reactor. All of the otheractions taken were as in Example 1. This gave a stable, coarse-particle(Coulter LS 230; Dw=2.57 μm) coagulate-free dispersion with a solidscontent of 49.3% and a viscosity (Brookfield viscometer 20° C., 20 rpm)of 275 mPas. The dispersion powder was prepared as in Example 1.

Comparative Example 2

The dispersion was prepared as in Example 1, the entire amount of 900 gof the 20% strength by weight aqueous solution of the partiallyhydrolysed polyvinyl acetate with a degree of hydrolysis of 88 mol % andwith a Höppler viscosity of 4 mPas for the 4% strength solution beingused as a feed into the pressure reactor. All of the other actions takenwere as in Example 1. This gave an incompletely polymerized, verycoarse-particle (Coulter LS 230; Dw=4.37 μm) high-coagulate dispersionwith a solids content of 32.1% and a viscosity (Brookfield viscometer20° C., 20 rpm) of 75 mPas. The dispersion powder was prepared as inExample 1.

Comparative Example 3

The dispersion was prepared as in Example 1, the entire amount (900 g)of the 20% strength by weight aqueous solution of the partiallyhydrolysed polyvinyl acetate with a degree of hydrolysis of 88 mol % andwith a Höppler viscosity of 4 mPas for the 4% strength solution, andalso the total amount of styrene, 1,3-butadiene and tert-dodecylmercaptan being used as a feed into the pressure reactor. All of theother actions taken were as in Example 1. This gave an incompletelypolymerized, coarse-particle (Coulter LS 230; Dw=3.72 μm) high-coagulatedispersion with a solids content of 26.1% and a viscosity (Brookfieldviscometer 20° C., 20 rpm) of 63 mPas.

Redispersion Behaviour of the Polymer Films

The dispersions from the examples mentioned were used to produce filmsof 0.2 mm thickness on glass plates and the films were dried at 105° C.for 15 minutes. To check film redispersibility one droplet of water wasapplied using a pipette at room temperature to a homogeneous part ofeach film to be tested, and after allowing 60 seconds for the waterdroplet to take effect, a fingertip was used to rub the same area untilthis area of the glass plate was free from film, or the film fragmented,or remained intact.

The redispersibility of the polymer films was assessed using thefollowing evaluation scale:

Grade 1: Rubbing lightly immediately redisperses the film, or itredisperses spontaneously;

Grade 2: Rubbing redisperses the film, but some film fragments may bedifficult to disperse;

Grade 3: Vigorous rubbing was required to redisperse the film, withfragmentation of the film;

Grade 4: Even after prolonged vigorous rubbing the film does notredisperse, but fragments.

Determination of the Sedimentation Behaviour of the Powder (TubeSedimentation)

To determine sedimentation behaviour, 50 g of each dispersion powderwere redispersed in 50 ml of water, then diluted to 0.5% solids content,and the height of settled solids was measured for 100 ml of thisredispersion poured into a graduated tube, settlement being measuredafter 1 hour.

Determination of Blocking Resistance

To determine blocking resistance, the dispersion powder was placed in aniron pipe (diameter: 5 cm) with a thread, and then subjected to a loadfrom a metal ram (weight: 3 kg). The application of the load wasfollowed by storage for 16 hours at 50° C. in a drying cabinet. Aftercooling to room temperature, the powder was removed from the tube andresistance to blocking was determined qualitatively by crushing thepowder. Blocking resistance was classified as follows:

1=very good blocking resistance

2=good blocking resistance

3=satisfactory blocking resistance

4=not resistant to blocking—powder after crushing no longerfree-flowing.

Determination of Air Content in the Mortar

A DIN mortar to DIN 1164 was mixed with the formulation below with awater-cement factor W/C of 0.45 and a polymer-cement factor P/C of 0.15:

PZ-35F Portland cement 900 g Standard sand 2700 g S-860 Siliconeantifoam (Wacker Chemie) 7.2 g Dispersion powder 135 g Water 405 g

Air content was determined using DIN 18555 Part 2.

Determination of Cement Stability

A cement mix was prepared with the following formulation:

Portland cement 82.5 g Calcite (CaCO)₃ 10-40 mm 75 g Quartz sand 200-500mm 128 g Dispersion powder 15 g Water 85 g

The workability of the cement mix was observed over a period of 2 hoursand assessed qualitatively.

The test results are given in Table 1.

TABLE 1 Tube Film sedimen- Air con- redispers- tation 1 Blocking tent inCement Example ibility h [cm] resistance mortar stability Ex. 1 Grade 10.1 2 6% 2 h Ex. 2 Grade 1 0.1 2 4% 2 h Ex. 3 Grade 4 0.2 2 5% 2 h Ex. 4Grade 2 0.3 2 5% 1.5 h   Ex. 5 Grade 1 0.2 2 7% 2 h Ex. 6 Grade 1 0.1 26% 2 h Ex. 7 Grade 2 0.3 2 5% 2 h Comp. Grade 4 0.4 2 4% 15 Ex. 1minutes Comp. Grade 4 1.1 3 — 15 Ex. 2 minutes Comp. Grade 4 3.1 3 — 15Ex. 3 minutes

The procedure according to the invention can give dispersion powdersbased even on copolymers of hydrophobic comonomers, such as styrene andbutadiene. The powders have very good redispersibility (tubesedimentation) and very good performance (cement stability). If thecontent of protective colloid is entirely used as initial charge orentirely used as feed during the emulsion polymerization, both theredispersibility of the resultant powders and their performance areunsatisfactory.

We claim:
 1. Process for preparing protective-colloid-stabilizedvinlyaromatic-1,3-diene copolymers in the form of their aqueous polymerdispersions or in the form of their water-redispersible powders, byemulsion polymerization of a mixture comprising at least onevinylaromatic monomer and comprising at least one 1,3-diene monomer inthe presence of protective colloid and, optionally, drying the resultantpolymer dispersions, wherein some of the protective colloid is used asan initial charge and some of the protective colloid is used as a feed,and wherein the weight ratio of protective colloid initially present tomonomers initially present is from 0.1:1 to 0.9:1, and wherein from 2 to90 percent by weight of total protective colloid is used in the initialcharge, and the remainder is metered as said feed.
 2. Process accordingto claim 1, wherein use is made of mixtures with from 20 to 80% byweight of vinylaromatic and from 20 to 80% by weight of 1,3-diene, wherethe mixtures may, optionally, also comprise other monomers and the percent by weight data always give a total of 100% by weight.
 3. Processaccording to claim 2, wherein styrene and 1,3-butadiene arecopolymerized.
 4. Process according to any of claims 1 to 3, wherein theprotective colloid used comprises one or more selected from the groupconsisting of fully and partially hydrolysed polyvinyl acetates,partially hydrolysed hydrophobicized polyvinyl acetates,polyvinylpyrrolidones, starches, celluloses, carboxymethyl-, methyl-,hydroxyethyl- and hydroxypropylcellulose, proteins, poly(meth)acrylicacid, poly(meth)acrylamide, polyvinylsulphonic acids,melamine-formaldehydesulphonates, naphthalene formaldehydesulphonates,styrene-maleic acid copolymers, vinyl ether-maleic acid copolymers, anddextrins.
 5. Process according to claim 4, wherein the polymerization iscarried out in the presence of partially hydrolysed polyvinyl acetatewith a degree of hydrolysis of from 80 to 95 mol % and with a Höpplerviscosity of from 1 to 30 mPas.
 6. Process according to claim 5, whereinfrom 0.05 to 10% by weight, based on the total weight of the monomermixture, of auxiliary monomers selected from the group consisting ofethylenically unsaturated mono- and dicarboxylic acids, ethylenicallyunsaturated carboxamides, ethylenically unsaturated carbonitriles, mono-and diesters of fumaric acid or of maleic acid, ethylenicallyunsaturated sulphonic acids and salts of these, precrosslinkedcomonomers having two or more ethylenic unsaturated bonds,postcrosslinking comonomers, epoxy-functional comonomers,silicon-functional comonomers and comonomers having hydroxyl or COgroups are also copolymerized.
 7. Process according to claim 6, whereinthe polymer dispersion is dried by spray drying, if desired after addingfurther protective colloids.
 8. A process for preparingprotective-colloid-stabilized vinylaromatic-1,3-diene copolymers in theform of their aqueous polymer dispersions or in the form of theirwater-redispersible powders, comprising emulsion polymerizing a mixturecomprising at least one vinylaromatic monomer and at least one 1 3-dienemonomer in the presence of a protective colloid to produce a polymerdispersion, and optionally drying the polymer dispersion, wherein someof the protective colloid is included in the initial charge and some ofthe protective colloid is added as a feed, and wherein thepolymerization takes place in the absence of emulsifier, and wherein theweight ratio of protective colloid initially present to monomersinitially present is from 0.1:1 to 0.9:1, and wherein from 2 to 90percent by weight of total protective colloid is used in the initialcharge, and the remainder is metered as said feed.
 9. The process ofclaim 8, wherein mixture comprises from 20 to 80% by weight ofvinylaromatic monomer(s), from 20 to 80% by weight of 1,3-dienemonomer(s), and optionally other monomer, and the per cents by weight ofthe individual monomers total 100%.
 10. The process of claim 8, whereinstyrene and 1,3-butadiene are copolymerized.
 11. The process of claim 8,wherein the protective colloid used comprised at least one of fullypartially hydrolysed polyvinyl acetates, partially hydrolysedhydrophobicized polyvinyl acetates, polyvinylpyrrolidones, starches,celluloses, carboxymethyl-, methyl-, hydroxyethyl- andhydroxypropylcelluloses, proteins, poly(meth)acrylic acid,poly(meth)acrylamide, polyvinylsulphonic acids,metaminsformaldehydesulphonates, naphthalene, formaldehydesulphonates,styrene-maleic acid copolymers, vinyl ester-maleic acid copolymers, anddextrins.
 12. The process of claim 8, wherein the polymerization iscarried out in the presence of partially hydrolysed polyvinyl acetatewith a degree of hydrolysis of from 80 to 95 mol % and with a Höpplerviscosity of from 1 to 30 mPas.
 13. The process of claim 8, wherein from0.05 to 10% by weight, based on the total weight of the monomer mixture,of at least one of ethylenically unsaturated mono- and dicarboxylicacids, ethylenically unsaturated carboxamides, ethylenicallyeunsaturated carbonitrile mono- and diasters of fumaric acid or ofmaleic acid, ethylenically unsaturated sulphonic acids and salts ofthese, precrosslinked comonomers having two or more ethylenicunsaturated bonds, postcrosslinking comonomers, epoxy-functionalcomonomers, silicon-functional comonomers and comonomers having hydroxylor CO groups are also capolymerized.
 14. The process of claim 8, whereinthe polymer dispersion is dried by spray drying, optionallt after addingfurther protective colloids.
 15. In a cementitious composition employinga redispersible polymer powder prepared by emulsion polymerization offrom 20 to 80 weight percent of vinyl aromatic monomer, from 80 to 20weight percent of a 1,3-diene monomer, and optionally up to 30 weightpercent of other comonomers, the weight precents based on the totalweight of the redispersible polymer, the improvement comprisingselecting as said radispersible polymer powder the redispersible polymerpowder prepared by the process of claim
 1. 16. In a cementitiouscomposition employing a redispersible polymer powder prepared byemulsion polymerization of from 20 to 80 weight percent of vinylaromatic monomer, from 80 to 20 weight percent of a 1,3-diene monomer,and optionally up to 30 weight percent of other comonomers, the weightpercents based on the total weight of the redispersible polymer, theimprovement comprising selecting as said redispersible polymer powderthe redispersible polymer powder prepared by the process of claim
 3. 17.In a cementitious composition employing a redispersible polymer powderprepared by emulsion polymerization of from 20 to 80 weight percent ofvinyl aromatic monomer, from 80 to 20 weight percent of a 1,3-dienemonomer, and optionally up to 30 weight percent of other comonomers, theweight precents based on the total weight of the redispersible polymer,the improvement comprising selecting as said redispersible polymerpowder the redispersible polymer powder by the process of claim
 8. 18.In a cementitious composition employing a redispersible polymer powderprepared by emulsion polymerization of from 20 to 80 weight percent ofvinyl aromatic monomer, from 80 to 20 weight percent of a 1,3-dienemonomer, and optionally up to 30 weight percent of other comonomers, theweight precents based on the total weight of the redispersible polymer,the improvement comprising selecting as said redispersible polymerpowder the redispersible polymer powder by the process of claim 12.